Analysis of the aging effects on the viscoelasticity of alginate gels

Abstract

The effect of aging on the mechanical behaviour of ionically cross-linked alginate gels is studied in detail. Relaxation experiments upon both unconfined compression and torsion are performed on samples at different aging times. The elastic moduli of the gel are found to increase with the aging time, whereas the internal (constitutive) mechanism of the relaxation of the solid component of the gel is found to be unaffected by aging. It is demonstrated that the Linear Visco-Elastic Stress/Diffusion Coupling model [D. Larobina, F. Greco, J. Chem. Phys., 2012, 136, 134904], recently developed by two of the present authors, is able to quantitatively reproduce the experimental data for differently aged samples, at early-to-intermediate relaxation times. Moreover, it is shown that the gel always undergoes a spontaneous expulsion of water (syneresis) and some spontaneous deformation for a sufficiently long observation time, even in the absence of any externally imposed strain. The latter phenomenology progressively slows down with increasing of the gel age. By proper time shifting of the late relaxation decays, i.e., by properly defining an "effective time", master curves can be obtained in all cases, with all data pertaining to differently aged samples collapsing on a single relaxation law for each deformation history. The dependence of the shift factors on the aging time is found to follow a power law behavior, with an exponent of 1.39.

Anomalous Aging and Stress Relaxation in Macromolecular Physical Gels: The Case of Strontium Alginate

“…In view of these macroscopic stress-relaxation data, we suggest that two mechanisms are at play: the first one, at earlyto-intermediate times, should be ascribed to local relaxations in the gel network (ultimately due to the finite lifetime of physical junctions); the second one, at long time, should instead result from more collective rearrangements, related to gel syneresis and aging. The presence of two regimes in stress relaxation had been already reported for similar gels, 34,36 but we here provide the first clear identification of such regimes, which are quantitatively shown to obey different functional forms (earlylogarithmic and late-stretched exponential).…”

“…Indeed, a rough estimate of the characteristic time of diffusion can be obtained from the typical value of the diffusion coefficient (D) of copper ion into calcium alginate gels 58 and gel half-thickness (l): We can therefore explicitly infer that the observed contraction is due to a spontaneous rearrangement of the alginate network and not to ions diffusion inside the gel, with ensuing junction formation. 34 Consistent with this observation, focus of our experimental investigation is on a time window in which ion diffusion is negligible, and therefore we choose the smallest waiting time, t w = 6 h, much larger than τ diff .…”

Enhanced microscopic dynamics in mucus gels under a mechanical load in the linear viscoelastic regime

“…We probe the gel response on a wider range of time scales in stress relaxation tests, where a step strain of amplitude γ0 is applied at t = 0 and σ(t), the time evolution of the stress needed to maintain such a deformation, is followed for up to 2000 s. Figure 1b shows the relaxation modulus G(t) = σ(t)/γ0 for four strain amplitudes ≤ 20%. The decay of G(t) is close to logarithmic, confirming a wide distribution of relaxation times, a behavior similar to that reported in other soft solids (alginate gels (41)(42)(43), granular media under compression (44,45), colloidal glasses in creep tests (46,47)). While the applied strain changes by a factor of 40, all G(t) curves superimpose, indicating linear viscoelastic behavior up to γ0 = 20%.…”

Characterization of Polyethylene Glycol–Reinforced Alginate Microcapsules for Mechanically Stable Cell Immunoisolation

“…The hybrid MicroMix (MM) displayed very slight shrinking compared to ALG capsules (average ratio of final to initial diameter: MM‐DTT, 0.981, p = 0.0011; MM‐PEGSH, 0.958, p = 0.0240). The spontaneous syneresis of ALG under quiescent conditions has been described before, and it appears the incorporation of a PEG‐network provides a counter‐balance to abrogate the ALG shape change …”

“…ALG‐only, MM‐DTT, and MM‐PEGSH showed significantly improved total resistance to stress, instantaneous recovery, and long‐term recovery when the 7‐day capsules were compared to the 2‐day capsules (Figure 2A–C, Supporting Information). This increased elasticity has been reported before and can be due to spontaneous thermodynamically favored ALG chain rearrangements or the sequestration of additional calcium . Alginate tends to not only increase junctional density, but also develop ordered fibrillar structures over time .…”